How can a moon-sized creature evolve?

Question

I wanted to write the evolutionary history of one of the creatures in my story based on scientific principles. This creature is the size of the moon and can live in space. I initially used the limited knowledge I have about biology.

The animal I chose for evolution originally lived on a planet much larger than Earth, about a hundred times its size, mostly composed of water with organisms resembling those on Earth and double the gravity of Earth.

Initially, it was a small marine creature, relatively small (the size of a sardine), inhabiting the ocean. It formed a symbiotic relationship with a type of algae that produced energy through photosynthesis and consumed the creature's dead skin, sharing the energy for reproduction, survival, and nourishment. "Due to the absence of natural predators and an abundance of food, it began to take on a snake-like form with multiple limbs, growing larger with each generation.

Later, it developed a symbiotic relationship with bacteria that produced oxygen. These bacteria began to inhabit its pores, feeding on the dead algae on its skin, the heat from the Earth, and its own body. They produced the oxygen necessary to support its life. Consequently, the creature continued to grow larger, and the heat generated by its body became the primary source of warmth for the bacteria, independent of the Earth's heat. Finally, this creature became composed of several parts: internal organs, bones, and scales. Above these were the algae and bacteria that lived within its body. On top of them was a membrane with multiple functions, serving to protect both the creature and the organisms within it from the heat generated by its body. The creature passed this heat through specialized channels outside the membrane, preventing it from affecting the algae and bacteria.

Regarding its ability to live in outer space, I considered making it feed on cosmic dust and radiation.

My question :understand that my concept may contain errors, gaps, or missing information. Can you point them out and provide scientific solutions while keeping the general appearance of the creature intact?

Answer 1

I am afraid your moon-sized creature is way outside the realm of hard science.

List of Porblems:

How does it move?

Does it move? Moving is hard in space because there is nothing to push off. Your only options are to throw propellant behind you or maybe use a solar sail.

How did it get into space?

Either it ate the planet it lived on (how?) or it has rocket thrusters. Where did it find the thrusters?

What does it eat?

Space is big and empty. There is no food in space. That's why astronauts bring their own food.

How does it stay together?

Big things tend to collapse under their own weight. At large scales matter behaves like a liquid and condenses into spherical blobs, i.e planets and moons. Your monster will be the same. On top of that, the pressure at the core causes the moon to heat up to about 1,500 C which is too hot to live.

Solution

The most believable moon-sized creature is some form of photosynthetic colony of bacteria/plants that extends in a membrane over the moon's surface. It never evolves to try and move the moon. Moving the moon is impossible and there is no reason to try, since the colony gets all the sustenance it needs from the sunlight.

HOWEVER this idea of a planet-spanning colony somewhat contradicts the guiding principle of evolutionary theory, which is survival of the fittest. Typically an equilibrium is required to prevent one creature overpopulating and starving itself and everything else. See algal blooms and fish kills. So you need to explain why the uber-colony does not continue to reproduce unchecked and exhaust its food source.

I am tempted to say this is not a problem with plants. You never hear of a whole forest falling down if there is nothing there to eat the leaves. Perhaps that's true. Or perhaps the trees all die when the soil gets exhausted with nothing there to make new soil (dung). This is your homework to research in your own time.

Answer 2

A very much smaller example is the Pando or clonal colony of the quaking aspen. This is a single living thing weighing 6 million Kg. This coexists with the creatures that live with it. However, suppose you had something that had some more animal-like properties - it could see or sense, communicate between its nodes, and react to a threat, perhaps using the fauna that lived within it. The co-dependent fauna might evolve to serve the needs of the host. They might do this independently, much as our gut bacteria within us do, or they might act cooperatively with the host in a much more complicated way.

The whole system is on a pre-existing planet, but only occupying the surface to a depth suitable to life. I can see no reason why it might want to change the orbit of the planet it lived on. If it thought of itself as the only intelligence, it might not even be curious about what lay outside. It would be interesting to see how it would handle meeting another intelligence.

Answer 3

This would be an extraordinary feat and it would require extraordinary control of matter and energy, and it might need extraordinary physics that we do not know of.

But there could be something that comes close: the organism has taken over the whole moon, and it consumes the moon's resources as much as possible and leaves behind base rock as a skeleton. If it also does photosynthesis, it is basically a moon-sized plant that has roots that extend kilometers (miles) below the surface, and maybe giant tubulars and such that can take use of the rock to support the structure.

I hope you get the water somewhere, though, or something that can be used instead of water. Organism of this size need it a lot. It does not necessarily need any water outside it, so it can freely hog it all and use it as it needs.

Answer 4

Your key problem? Individual complex organisms don't 'evolve'. The species they are a member of over time do evolve

Your organism is a symbiote. That means the individual species involved in the symbiotic relationship can in theory 'evolve'(change over time in response to environmental stresses). But not the 'Creature' itself. Even then the problem remains that species in a symbiotic relationship are very much 'bound together' which somewhat limits the opportunities for divergence unless for some reason there's pressure placed on the symbiotic relationship that forces a % of the population of one partner species to abandon the symbiosis and evolve independently.